Class X: Science, Board Paper 2009, Set-2

General Instructions:
(i) The question paper comprises of two sections, A and B. You are to attempt both the sections
(ii) All questions are compulsory
(iii) There is no overall choice. However, internal choice has been provided in all the three questions of five marks category. Only one option in such questions is to be attempted
(iv) All questions of Section A and all questions of Section B are to be attempted separately
(v) Questions number 1 to 6 in Section A and 17 to 19 in Section B are short answer type questions. These questions carry one mark each
(vi) Questions number 7 to 10 in Section A and 20 to 24 in Section B are short answer type questions and carry two marks each
(vii) Questions number 11 to 14 in Section A and 25 and 26 in Section B are also short answer type questions and carry three marks each
(viii) Questions number 15 and 16 in section A and question number 27 in section B are long answer type questions and carry five marks each

Tooth enamel, which is made of calcium phosphate, is sensitive to pH.It does not dissolve in water (with neutral pH). However, it gets corroded when the pH inside the mouth is below 5.5. Hence, tooth decay starts when the pH of mouth is lower than 5.5.

Different
media have different refractive indices. Due to this difference in
the refractive indices, the speed of a light ray in one medium is
different from its speed in another medium. Hence, a light ray bends
on moving from one medium to another.

Q6

Take
down this diagram onto your answer book and complete the path of ray.

The
reaction in which a substance gains oxygen or loses hydrogen is
called an oxidation reaction.

For
example, when copper (Cu) is heated in the presence of air, it gets
oxidised to form copper oxide (CuO). The chemical equation involved
in this reaction can be represented as

Since
heat energy is absorbed in this process, this reaction is endothermic
nature.

[Please
note that oxidation can be either exothermic or endothermic depending
on the reaction. However, in the NCERT textbook, all the given
oxidation reactions are endothermic in nature]

Q8

A
compound which is prepared from gypsum has the property of hardening
when mixed with a proper quantity of water. Identify the compound.
Write the chemical equation for its preparation. For what purpose is
it used in hospitals?

(b) Blackening (deposition of unburnt carbon particles) of the
cooking vessels shows that incomplete combustion of the gas is taking
place. As complete combustion is not taking place, the maximum amount
of energy that can be obtained from the fuel cannot be obtained,
i.e., gas is being wasted. Limited supply of air results in the
incomplete combustion of saturated hydrocarbons. Hence, the air holes
of a gas burner have to be adjusted so that a sufficiently
oxygen-rich mixture is burnt and a clean blue flame is
obtained.

(c) Synthetic detergents are non-biodegradable in nature and
cannot be decomposed by micro-organisms. Therefore, they remain as
they are for a long time in nature, thereby causing water pollution.
Hence, the use of synthetic detergents causes pollution of water.

Q12

What
is meant by ‘refining of metals’? Describe the
electrolytic refining of copper with a neat labelled diagram.

The
metals obtained by different processes of reduction, except
electrolytic reduction, contain various impurities. The removal of
these impurities to obtain pure metals is called refining of metals.

Electrolytic
refining of copper

In
this process, the anode is made of impure
metal and cathode is made of pure metal. The acidified solution of
copper sulphate used is known as the electrolyte.

When
electric current is passed through the electrolyte, the positively
charged metal ions present in the electrolyte get attracted towards
the negatively charged cathode. When metal ions come near the
cathode, they gain electrons from it and get deposited there. Hence,
as time increases, the cathode becomes thicker and thicker.

As
the metal ions from the electrolyte move towards the cathode, an
equal amount of ions is released by the anode into the solution.
Therefore, as time increases, the size of the block of impure metal
keeps decreasing i.e., the anode becomes thinner and thinner.

The
soluble impurities present in the impure metal block go into the
electrolytic solution. On the other hand, the insoluble impurities
settle down at the bottom of the container (below
the anode) and are known as anode mud
(as shown in the figure).

Figure:
Electrolytic refining of copper

Q13

Two
resistors, with resistances 10 Ω
and 15 Ω, are
to be connected to a battery of emf 12 V so as to obtain:

(i) minimum
current (ii) maximum current

(a) Describe
the mode of connecting the resistances in each case.

(b) Calculate
the strength of the total current in the circuit in each case.

(a) The two criteria used by Mendeleev to classify the
elements in his periodic table are:

Atomic mass: elements are arranged in the order of increasing
atomic masses.

The formulae of the hydrides and the oxides formed by the various
elements.

(b) Mendeleev’s Periodic Law states that the properties
of elements are a periodic function of their atomic masses. This
means that if elements are arranged in the increasing order of their
atomic masses, then their properties will get repeated after regular
intervals.

(c) Mendeleev positioned hydrogen in the first column above
the alkali metals. He did so because hydrogen and alkali metals have
similar properties. For example, hydrogen reacts with halogens,
oxygen, and sulphur to form compounds whose formulae are similar to
those of alkali metals.

However, hydrogen also resembles halogens in many ways. Like
halogens, hydrogen is a gas, and exists as a diatomic molecule (H2).
It forms covalent compounds like halogens and unlike alkali metals.
Hence, it can also be placed above the halogen group.

Therefore, Mendeleev was not able to explain the positioning of
hydrogen. In other words, the position of hydrogen in Mendeleev’s
periodic table was not justified. This was one of the limitations of
Mendeleev’s periodic table.

(d) (i) As we move across the periodic table from left to
right in a period, atomic size decreases as effective nuclear charge
increases.

(ii) As we move down a group in the periodic table, atomic size
increases as new shells are being added.

OR

(a) Mendeleev left some gaps in his periodic table. He had
predicted that some elements were yet to be discovered.
He left these gaps deliberately so that these undiscovered
elements could get a place in his periodic table when they would be
discovered in the future. Hence, he left some gaps in his Periodic
Table.

(b) Three
limitations of Mendeleev’s classification are as follows:

(i)
The position of hydrogen was not justified in Mendeleev’s
periodic table.

(ii) Although Mendeleev arranged the elements in the increasing order
of their atomic masses, there were instances where he placed an
element with a slightly higher atomic mass before an element with a
slightly lower atomic mass. For example, cobalt, whose atomic mass is
58.9, was placed before nickel whose atomic mass is 58.7.

(iii) The discovery of isotopes revealed another limitation of
Mendeleev’s periodic table. Atoms of the same elements having
different number of neutrons are called isotopes. For example, the
isotopes of chlorine are Cl-35 and Cl-37. They have the same chemical
properties, but different atomic masses. Since Mendeleev’s
periodic table was based on the atomic masses of elements, isotopes
would be placed in different columns, despite the fact that they
represent the same element.

(c) The atomic numbers of elements increase by one on moving
from left to right in the periodic table. Therefore, the number of
electrons also increases by one.

Hence, on moving across the periodic table from left to right, the
valence electrons will increase by one, while the valence shell
remains the same.

Q16

(a) What is a magnetic field? How can the direction of magnetic
field lines at a placed by determined?

(b) State the rule for the direction of the magnetic field
produced around a current carrying conductor. Draw a sketch of the
pattern of field lines due to a current carrying conductor. Draw a
sketch of the pattern of filed lines due to a current flowing through
a straight conductor.

OR

(a) What is a solenoid? Draw a sketch of the pattern of field
lines of the magnetic field through and around a current carrying
solenoid.

(b) Consider a circular loop of a wire lying in the plane of the
table. Let the current pass through the loop clockwise. Apply the
right hand rule to find out the direction of the magnetic field
inside and outside the loop.

(a) Magnetic field is the region around a magnet in which a
magnetic material experiences a force because of that magnet.

The direction of magnetic field lines at a place can be determined
by placing a magnetic compass in the field. The direction along which
the magnetic needle of the compass aligns itself gives the direction
of the magnetic field lines.

(b) The direction of the magnetic field produced around a
current-carrying conductor is determined by the right-hand thumb
rule. According to this rule:

If one holds a current-carrying conductor with the right hand, such
that the thumb points in the direction of the current flowing through
the conductor, then the other fingers will wrap around the conductor
in the direction of the field lines of the magnetic field due to the
conductor.

OR

(a) Solenoid: A solenoid is a coil of several circular turns
of insulated conducting wire, wrapped closely in the shape of a
cylinder.

(b) The direction of the magnetic field inside the loop will
be in the downward direction, i.e., from the top of the table towards
its bottom.

The direction of the magnetic field outside the loop will be in the
upward direction, i.e., from the bottom of the table towards its top.

[The direction of the magnetic field produced around a
current-carrying conductor is determined by the right-hand thumb
rule. According to this rule:

If one holds a current-carrying conductor with the right hand, such
that the thumb points in the direction of the current flowing through
the conductor, then the other fingers will wrap around the conductor
in the direction of the field lines of the magnetic field due to the
conductor.]

Nastic movements are non-directional movements that show
response towards stimuli such as light, temperature, humidity, etc.
The direction of nastic movements is independent of the direction of
stimulus. Opening and closing of flowers is an example of photonastic
response.

Curvature movements involve curving or bending of a plant in
response to any stimuli. The curving of the stem tip towards light,
due to the hormone auxin, is an example of curvature movement.

Q21

What
is water harvesting? Mention any two water harvesting structures.

Rainwater
harvesting is the method of collecting rainwater for future use.
Harvesting of rainwater is essential for ensuring the availability of
usable water. The underlying concept is to make rainwater percolate
below the ground, so that the level of groundwater gets replenished.

Khadins
and Nadis are two water harvesting structures that are
prominent in Rajasthan. Small pits, lakes, earthen dams, sand and
limestone reservoirs, and rooftop water collecting units are some of
the structures that are used for water harvesting.

Q22

Why
must we conserve our forests? List any two causes for deforestation
taking place.

We must
conserve our forests to preserve biodiversity (range of different
life forms) so as to avoid the loss of ecological stability. A large
number of tribes live in and around forests. If forests are not
conserved effectively, then these tribes may lose their habitat. If
forests are not managed properly, then water resources, quality of
soil, and even the amount of rainfall in a particular region may get
affected. In simple words, without forests, life will become
extremely difficult for human beings

The two
main causes of deforestation are

Industrialization

Industries
regularly derive several raw materials from forests. This leads to
large scale deforestation.

Urbanization

The
increase in urbanization has a direct impact on forests. Since
urbanization requires large open areas, a large number of trees are
felled for this purpose.

Q23

(a) Distinguish between renewable and non-renewable sources of
energy.

The
gas made from the anaerobic digestion of
agricultural and animal wastes is called biogas. When the biomass
(agricultural and animal wastes) is decomposed in the absence of
oxygen, it releases biogas.

Biogas is considered
as an ideal fuel because it:

(i) Has a high
calorific value

(ii) Produces no
smoke on burning

(iii) Burns smoothly
without any explosion

(iv) Leaves no
residue on combustion

Q25

What
are sexually transmitted diseases? Name four such diseases. Which one
of them damages the immune system of human body?